1. Field of the Invention
The present invention relates to a WDM (Wavelength Division Multiplex) terminal device and a WDM network. More particularly, the present invention relates to a WDM terminal device accommodating a client signal in a WDM network, and the WDM network.
2. Description of the Related Art
According to a related art, accommodation of a client signal to a WDM system is performed separately for each wave, by use of an optical fiber corresponding to the wave. All the client signals are initially converted from a light-signal format to an electric-signal format, and, then, converted from the electric-signal format to a light-signal format, by a transponder, inside or near the WDM system. Subsequently, all the client signals are accommodated in the WDM system. The above-described steps are performed because a wavelength of each client signal supplied to the WDM system is different from a wavelength of each client signal inside the WDM system.
Two types of methods are taken for accommodating a plurality of client signals located at a distant place, according to the related art. According to a first method shown in FIG. 1, client signals to be accommodated in a WDM system are transmitted separately by use of optical fibers whose number is equal to the number of the client signals, from the distant place to the WDM system. Subsequently, the client signals are accommodated in WDM terminal devices 10 or 11, or a WDM-ADM (Add-Drop Multiplexer) device 12 of the WDM system, through a transponder.
On the other hand, according to a second method shown in FIG. 2, a WDM terminal device 15 of a small-capacity WDM system initially carries out wavelength multiplexing to client signals at a distant place. Subsequently, the WDM terminal device 15 transmits the client signals closer to a WDM system that accommodates the client signals at the end. Next, a WDM terminal device 16 of the small WDM system separates the client signals by each wave. Finally, the client signals are accommodated in the WDM terminal devices 10 or 11, or the WDM-ADM device 12 of the WDM system.
For example, a wavelength-multiplexed signal having tens of channels is transmitted or received between the WDM terminal devices 10 or 11, and the WDM-ADM device 12 at a 0.4 nm interval through a band whose wavelength is 1500 nm. On the other hand, a wavelength-multiplexed signal having several to ten odd channels is transmitted or received between the WDM terminal devices 15 and 16, at a 1.6 nm interval through a band whose wavelength is 1500 nm, for instance.
Additionally, in the related art, system setting and management are independently performed to each of a long-distance WDM system, a middle or short distance WDM system and a MUX (Multiplexer) of a SONET (Synchronous Optical Network) system by use of its own EMS (Element Management System) and data link. Therefore, in a case in which a client constructs a network by combining each system, complicated management of the entire network by use of a plurality of EMSs is necessary.
Additionally, according to the first method, optical fibers whose number is equal to the number of the client signals to be accommodated are necessarily placed or leased between the distant place and the WDM system. Thus, a cost of achieving the fist method is high.
Further, according to the second method, the WDM terminal devices 15 and 16 of the small-capacity WDM system are necessarily placed at two positions right before the distant place and the WDM system accommodating the client signals at the end. Consequently, a cost of achieving the second method is also high.